1. Field of the Invention
The present invention relates to a process for eliminating nitrile impurities from polymers which contain acyclic N-vinylcarboxamide and/or vinylamine units.
2. Discussion of the Background
U.S. Pat. No. 4,421,602 discloses polymers which contain N-vinylformamide and vinylamine units. The polymers are prepared by polymerizing N-vinylformamide and subsequently partially hydrolyzing the polvinylformamides by exposure to acids or bases.
EP-A-0 216 387 discloses that copolymers of 5-90 mol % of N-vinylformamide and 95-10 mol % of an ethylenically unsaturated monomer from the group comprising vinyl acetate, vinyl propionate, the C.sub.1 -C.sub.4 -alkyl vinyl ethers, N-vinylpyrrolidone, the esters, nitriles and amides of acrylic acid and methacrylic acid in at least partially hydrolyzed form, in which up to 100 mol % of the formyl groups can be eliminated from the polymer, are used as wet and dry strength enhancers for paper by being added in amounts of from 0.1 to 5% of the weight of dry fibers to the pulp before sheet formation.
Polymers containing vinylamine units are furthermore used as dispersants for pigments, as scale inhibitors for water treatment, as detergent additives, as flocculants, retention and dehydrating agents and as fixatives in papermaking. Polymers used for these purposes must not contain any physiologically unacceptable impurities. The monomers on which the polymers are based contain nitrile impurities from the preparation, such as lactonitrile or formylalanine nitrile. These impurities cannot be removed at reasonable expense, for example by distillation, in the preparation of, for example, N-vinylformamide on the industrial scale. Since the nitrile impurities do not contain double bonds, they are virtually unchanged in the free-radical polymerization of the N-vinylcarboxamides. As is disclosed in the abovementioned references, polymers containing vinylamine units are preferably prepared by hydrolyzing homo-or copolymers containing N-vinylcarboxamide units. The acid hydrolysis of poly-N-vinylcarboxamides contaminated with nitriles results in hydrogen cyanide which it is absolutely necessary to decompose before use of the polymers.
EP-B-0 032 248 discloses a process for the detoxification of cyanide-containing waste waters and reaction solutions by reaction with hydrogen peroxide in the presence of sulfur or sulfur-eliminating compounds such as sodium thiosulfate. However, polymers containing free sulfur are undesirable in papermaking.
EP-A-0 422 536 discloses that the action of hydrogen peroxide on carboxyl-containing polymers results in a reduction in the molecular weight of the polymers. The polymers which have been broken down by oxidation are used as additives to detergents.